Dinosaur Stampede, or Swimming?

Far from being a dinosaur stampede, as commonly suspected, Lark Quarry reveals evidence of dinosaurs swimming or wading across a shallow river, according to University of Queensland scientists.

Doctoral student Anthony Romilio took 3D images of thousands of small dinosaur tracks from the central-western Queensland site. “Many of the tracks are nothing more than elongated grooves, and probably formed when the claws of swimming dinosaurs scratched the river bottom,” says Romilio. “Some of the more unusual tracks include ‘tippy-toe’ traces – this is where fully buoyed dinosaurs made deep, near-vertical scratch marks with their toes as they propelled themselves through the water.”

Other tracks provide clearer foot morphology, indicating an animal wading while partly supported by the water.

Most of the prints at Lark Quarry are from very small dinosaurs, and Romilio’s supervisor, Dr Steve Salisbury of the School of Biological Sciences, says the fact that they were able to touch the bottom while swimming suggests the water was just 14 cm deep. “Other traces look similar but are made by larger animals, suggesting the water must have been 40 cm deep at the time, so we think the tracks record crossings over a few days or a week during which the depth fluctuated,” says Salisbury.

While the evidence of changing water levels casts doubt on the idea of a single herd of panicked dinosaurs, Salisbury notes that when water depth increased many smaller dinosaurs would have left no mark on the bottom at all, suggesting that the total numbers making the crossing must have been very large.

Many tracks have large spacings, which Salisbury and Romilio see as evidence that the animals were being carried with the current. However, Salisbury says not enough is known about the shape or size of the river outside the area where the tracks were recorded to know if the dinosaurs were crossing the river or using its flow to assist transportation.

In a paper in the Journal of Vertebrate Paleontology, Romilio and Salisbury also cast doubt on the idea that the smaller Lark tracks, dubbed Skartopus and Wintonopus, were produced by two species with toes of different lengths. “3D profiles of Skartopus tracks reveal that they were made by a short-toed trackmaker dragging its toes through the sediment, thereby elongating the tracks,” explains Romilio. “In this context, they are best interpreted as just another variant of Wintonopus.”